IP-based forwarding of bridged and routed IP packets and unicast ARP

ABSTRACT

Disclosed herein are methods of forwarding data over an IP network. The methods may include receiving a packet from a source host connected to the IP network, identifying the IP address of a destination host designated in the packet, determining the location on the IP network where the destination host designated by the packet is connected, without reference to the MAC address specified in the packet, by using location-identification information stored on the IP network, and forwarding the packet to the location on the IP network where the destination host is connected without reference to the MAC address specified in the packet. Also disclosed herein are related network devices implementing such techniques and operations, as well as IP networks which include such network devices.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to:

U.S. Provisional Pat. App. No. 61/900,228, filed Nov. 5, 2013, titled“NETWORK FABRIC OVERLAY”; and

U.S. Provisional Pat. App. No. 61/900,349, filed Nov. 5, 2013, titled“IP-BASED FORWARDING OF BRIDGED AND ROUTED IP PACKETS AND UNICAST ARP”;

each of which is hereby incorporated by reference in its entirety forall purposes.

TECHNICAL FIELD

This disclosure relates to computer networking apparatuses and tomethods and apparatuses for forwarding data on computer networks.

BACKGROUND

Modern data networks typically handle a tremendous and ever-increasingquantity of data transmission, and thus it is beneficial to implementtechniques and specialized hardware which may reduce the amount ofextraneous and/or unnecessary traffic flow in modern networkarchitectures. However, despite the need for efficiency, current networkarchitectures oftentimes employ various procedures which are far fromoptimal.

One such operation frequently used in traditional Layer 3 networks isthe so-called “address resolution protocol” or ARP. ‘ARP-ing’ istypically employed in both the bridging and routing context tofacilitate communication between hosts as follows:

Generally, the process of initiating communication between source anddestination hosts begins with the source host determining the IP addressof the intended destination host through, for example, a ‘domain nameservice’ (DNS) hosted on a network-accessible server. Once the correctIP address is identified, a source host operating in a traditional Layer3 network will decide whether a ‘bridging’ or ‘routing’ procedure willbe used for forwarding packets to the destination host by assessingwhether or not the destination host is located on the source host's ownsubnet (for example, by comparing the result of applying a subnet mask(255.255.255.0), to its own and the destination host's IP addresses).

If source and destination hosts are located on the same subnet andpackets are to be ‘bridged,’ between hosts, the source host will employARP to determine the MAC address of the destination host which is neededto label the IP packets for forwarding. To determine the MAC address viaARP, the source host sends an ARP packet out onto its local subnet. TheARP packet is a Layer 2 broadcast packet. The relevant fields of abroadcast ARP packet are schematically illustrated in FIG. 1. All hostson the local subnet receive the broadcast ARP packet and compare theirown IP addresses with that of the target IP address listed in thebroadcast ARP packet. The host on the local subnet having the IP addressin question signals that it is the correct destination host through anARP response packet it prepares by modifying the target MAC addressfield of the ARP packet with it's own MAC address. The relevant fieldsof an ARP response packet are schematically illustrated in FIG. 2. TheARP response packet is then forwarded back to the source host. Thesource host now has the destination MAC address it needs to properlylabel IP packets for forwarding to the intended destination host.

Again, this packet-forwarding procedure is known in the art as‘bridging’ and works for packet-forwarding between source anddestination hosts located on the same subnet. Note that in bridging, thesource host was able to identify the Layer 2 MAC address of thedestination host without employing the use of a router-type networkdevice. Further note that once the source host learns the correct MACaddress of the destination host, packets transmitted by the sourcearrive at the destination without intervening modification.

As stated above, if the source host determines that it is not connectedon the same subnet as the destination host, a packet forwardingprocedure known in the art as ‘routing’ is employed to forward packetsinstead of the ‘bridging’ procedure just described. Unlike bridging,routing does involve the use of a router (as its name implies), andfurthermore, unlike bridging, does result in the modification of theoriginal packet.

In a conventional routing procedure, since the source host hasdetermined that the intended destination host is not connected on itslocal subnet, the source host forwards packets by setting their Layer 3destination address field to the intended destination host's IP address,but setting their Layer 2 destination address field to that of therouter's MAC address. If the source host doesn't know the router's MACaddress, it first ‘ARPs’ for it by sending out a broadcast ARP requestpacket with Layer 3 destination address field set to the router's IPaddress. The router then responds with an ARP reply packet carrying therouter's MAC address in essentially the same manner described above withrespect to local hosts. As indicated, once the router's MAC address isknown to the source host, the source host may begin forwarding packetsto the destination host by labeling them with the destination host's IPaddress and the router's MAC address.

When the router receives packets labeled with the router's Layer 2 MACaddress, but another host's Layer 3 IP address, the router consults itsrouting table to forward the packets. If the routing table indicatesthat the destination IP address is on another directly attached subnet,the router will consult an ARP table to check whether it has the MACaddress of the host corresponding to the destination IP address. If itfinds the MAC address, the router rewrites the packet's Layer 2destination address field with this MAC address and forwards the packetto the destination host. If the router does not find the destinationhost's MAC address in its ARP table, the router ARPs for the destinationhost's MAC address before rewriting the packet's Layer 2 destinationaddress field and forwarding the packet.

However, when the router receives a packet with its Layer 2 destinationfield set to its own MAC address, but with its Layer 3 destination fieldset to an IP address which, according to its routing table, is not in adirectly attached subnet, the router determines if the destination hostis accessible through another router. If so, the first router forwardsthe packet to the second router, rewriting the packet's Layer 2destination address with this second router's MAC address. (If the firstrouter doesn't know the second router's MAC address, it ARPs for it, inthe same manner as the original source host used ARP to determine thefirst router's MAC address.) This process may repeat—and the packet maythus hop from router to router—until it arrives at a router having theintended destination host connected on one of its directly attachedsubnets (as indicated in that router's routing table).

Thus, a distinction between bridging and routing is typically maintainedin the operation of a traditional network. When a packet is bridged by anetwork device, it is forwarded by the device on the network withoutmodification of the original packet. This functionality is typicallyembodied in a device generally referred to in the art as a “switch.” A“router” type network device, as distinct from a “switch,” modifiespackets prior to forwarding them, as illustrated by the routingtechnique just described. Thus, when a packet's destination host is onthe same subnet as its source host, the packet is typically forwardedwithout modification via bridging, and when a packet's destination is ona different subnet than its source the packet is typically modified andforwarded via routing. In practice, it is oftentimes the case thatnetwork devices operate as both switches and routers, and thus thedistinction between ‘bridging’ and ‘routing’ results in more complicatednetwork devices which must typically have logic devoted to performingboth functions, as well as logic devoted to performing a determination,in the first place, of whether to bridge or to route each incomingpacket.

SUMMARY OF THE DISCLOSURE

Disclosed herein are methods of forwarding data over an IP network. Themethods may include receiving a packet from a source host connected tothe IP network, identifying the IP address of a destination hostdesignated in the packet, determining the location on the IP networkwhere the destination host designated by the packet is connected,without reference to the MAC address specified in the packet, by usinglocation-identification information stored on the IP network, andforwarding the packet to the location on the IP network where thedestination host is connected without reference to the MAC addressspecified in the packet. In some embodiments, thelocation-identification information may include a list matching one ormore host IP addresses with one or more locations on the IP networkwhere the hosts are connected.

Also disclosed herein are network devices for receiving packets from oneor more source hosts connected to an IP network and forwarding thepackets to one or more destination hosts connected to the IP network. Insome embodiments, the network devices may include logic for receiving apacket from a source host connected to said network device, logic foridentifying the IP address of a destination host designated in areceived packet, logic for determining the location on the network wherea destination host designated by a received packet is connected, withoutreference to the MAC address specified in the received packet, vialook-up of the destination IP address in a list oflocation-identification information stored on the network, and logic forforwarding the received packet to the network device which is saidlocation on the network.

Also disclosed herein are IP networks which include a first set ofmultiple network devices for connecting multiple hosts to the network,and a second set of multiple network devices for connecting together thefirst set of network devices. In some embodiments, the network devicesin the first set may include logic for receiving a packet from a sourcehost connected to said network device, logic for identifying the IPaddress of a destination host designated in a received packet, logic forattempting to determine the location on the network where a destinationhost designated by a received packet is connected, without reference tothe MAC address specified in the received packet, via look-up of thedestination IP address in a first list of location-identificationinformation stored on the network, logic for labeling a received packetwith said location, and logic for forwarding a received packet to anetwork device in the second set when said location is not the samenetwork device in the first set having received the packet. In someembodiments, the network devices in the second set may include logic forreceiving a packet from a network device in the first set, andforwarding the received packet to the network device in the first setwhich is the location of the destination host on the network designatedin the packet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically illustrates the format of an IP packet.

FIG. 1B schematically illustrates the format of an ARP packet.

FIG. 2 is a flowchart schematically illustrating a sequence ofoperations for forwarding packets generated by a host connected to an IPnetwork according to the disclosure provided herein.

FIG. 3 is a flowchart schematically illustrating a sequence ofoperations for network response to an ARP packet generated by a hostconnected to the network according to the disclosure provided herein.

FIG. 4 schematically illustrates a simple network made up of 12 enddevices which, following the “access-aggregation-core” (AAC) model, areconnected through an access tier, an aggregation tier, and a top-levelcore tier.

FIG. 5 schematically illustrates a particular example of a basicleaf-spine network.

FIGS. 6A-1, 6A-2, and 6B schematically illustrate how a simple exampleof a 3-tier leaf-spine network may be built from just 4-port switches.

FIG. 7 schematically illustrates a leaf-spine network wherein variousIP-based packet forwarding operations are performed.

DETAILED DESCRIPTION

The distinctions maintained between bridging and routing in atraditional network, as described above, typically result in variouscomplexities and inefficiencies in a standard implementation. Oneexample is the ARP procedure used to determine host MAC addresses. Asillustrated by the sequence described above, a significant disadvantageof employing the ARP procedure to determine host MAC addresses is thatARP request packets are broadcast to every host on a given subnet. Suchbroadcasts flood a network with traffic. In addition, depending on thetopological connectivity of the various subnets on a network, broadcastloops may result. Although, routing protocols based on spanning-treetype algorithms may be used to eliminate the broadcast loops, in sodoing, many optimal paths through the network's topology are oftentimeseliminated. Accordingly, it is desirable to avoid or minimize thegeneration of broadcast ARP packets on a network. Nevertheless, typicalnetworks as implemented in current datacenters, do broadcast ARPrequests, do eliminate loops using spanning-tree algorithms, etc., anddo not employ effective techniques to minimize or eliminate thebroadcasting problem associated with the ARP procedure.

To restate the issue another way: the current state of art is to forwardIP Packets using combo switch-router network devices based on theirdestination MAC addresses and VLAN IDs if the packets arrive at anetwork device carrying a destination MAC address different than that ofthe router's (or if routing is simply not enabled on the networkdevice), and otherwise, if the packet's destination MAC address doesmatch that of the switch/router (and assuming routing is enabled), theswitch/router forwards packets based on the destination IP addressesdesignated in the packets. However, as indicated above, a significantdownside of this approach is that, in the former case, a source hosttypically utilizes an ARP procedure to discover the MAC addresses of itsdesired destination host on the local subnet—leading to inefficientflooding on the local subnet and imposing a significant burden on theend hosts who aren't interested in the flooded traffic. Thus, in currentnetwork implementations ARP requests are typically flooded to all theend devices in the flood domain (often a VLAN), unnecessarily sappingthe processing power of the end devices on the local subnet. In fact, insome large modern datacenters, flooded traffic very frequently consumesa large portion of the potentially available server CPU processingpower.

More generally, it is desirable to eliminate the distinction betweenswitched/bridged IP packets (packets which carry the MAC address of thereceiving switch-router) and routed IP packets (packets which carry aMAC address other than that of the receiving switch-router) so thatpackets entering a network may be treated uniformly, regardless of theirultimate destination. For example, eliminating the foregoing distinctionallows the forwarding tables stored at network ingress points to have asmaller scale: e.g., devices that support both bridging and routingneeded to support two sets of tables. One which stores host IP addressesand another which stores host MAC addresses (the latter of which isadditionally problematic because they cannot be aggregated due to theirlacking a hierarchical format).

Thus, network devices may operate, whenever possible, by forwardingpackets based on the destination IP address (IPv4 or IPv6) designated inthe packets that they receive. Note that “network device” should beunderstood to encompass both switches and routers, and also comboswitch/routers (except where it is clear from the context that oneparticular type of device or another is being referred to), since thesame physical device typically implements both switching/bridgingfunctionality as well as routing functionality. As stated, IP-basedforwarding may be performed by network devices for IP packets, and also,in some cases, for non-IP packets (example ARP family of protocols). Incase of ARP packets, the network devices forward the packets based onthe IP address inside the ARP payload after examining the ARP opcode(request or reply). In some embodiments, in order to preserve externalsemantical behavior for the benefit of hosts and/or network devicesdesigned for legacy networks, although forwarding based on IP, thenetwork devices may note whether a packet would have been routed orbridged. In the case of bridging in a legacy network (e.g., packetreceived by a network device labeled with a MAC address other than thatof the network device, e.g., routing is disabled on a network device'singress interface, etc.), the network device forwards the packet basedon the IP address but does not perform the rewrite operations whichmight typically be associated with IP routing—rewrite of the source anddestination MAC address fields, decrementing the TTL, etc. may besuppressed. On the other hand, if packets are such that a legacy networkwould expect them to be routed, the packets would be forwarded based ontheir IP address and the typical routing rewrite operations would beperformed.

It is noted in the context of handling ARP request packets that variousembodiments of the IP-based forwarding techniques disclosed herein maybe particularly advantageous because: (i) they eliminate (orsignificantly reduces) one of the most common sources of broadcast orflooded traffic (which is especially important for cloud and data centernetworks); and (ii) they improve network scaling properties by allowingnetworks to operate with forwarding tables based on IP addresses alongwith local forwarding tables having the MAC addresses of locallyattached hosts, rather than operating with forwarding tables whichgenerally store an IP address and MAC address pair for all hosts/enddevices connected to the network. Accordingly, in various embodiments,the foregoing ARP forwarding technique may provide benefits in that itmay: (i) eliminate the need for external directory services, (ii) allowresolution of ARP requests in-line with regular packet flow to endhosts/devices, (iii) better distribute the burden of responding to ARPrequests to the end devices targeted by the ARP requests, (iv)efficiently provide opportunities for end devices to update their ARPcaches, (v) use remote station (top) and local station (bottom) tableefficiently, i.e. reduce/eliminate need for learning MAC addresses, and(vi) allow source IP learning based on conversations (triggered by ARP).

Accordingly, disclosed herein are methods, network devices, and IPnetworks for forwarding packets of data based on the IP address of thedestination host designated in the packets, rather than, and withoutreference to, the MAC addresses specified in the packets. Generallythese packets are IP packets but, as described above, ARP requestpackets may also be forwarded in this manner since they do provide adestination IP address in their payloads, and by doing so, subnet-widebroadcast of ARP request packets may be avoided. For instance, certainsuch method embodiments are schematically illustrated by the flowchartin FIG. 2. As shown in the figure, in some embodiments, a method 200 offorwarding data over an IP network may include: receiving a packet froma source host connected to the IP network in operation 210, identifyingthe IP address of a destination host designated in the packet inoperation 220, determining in operation 230 the location on the IPnetwork where the destination host designated by the packet isconnected, without reference to the MAC address specified in the packet,by using location-identification information stored on the IP network,and, finally, in operation 240, forwarding the packet to the location onthe IP network where the destination host is connected, the forwardingto said location done without reference to the MAC address specified inthe packet.

The location-identification information may reside in a database whichmay be implemented, for example, as a list which matches one or morehost IP addresses with one or more locations on the IP network where thehosts are connected. Depending on the embodiment, such a list, or moregenerally, such a database of location-identification information, maybe associated with (e.g., stored locally on) the network devicereceiving the packet as it enters the IP network—typically the firstnetwork device initially encountered by the packet when it reaches theIP network after it issues from the source host. In other embodiments,such a list or database may be associated with (e.g., stored on) anothernetwork device, or multiple other network devices on the IP network, orthe database/list may be distributed across multiple network devices, orstored in-whole on one network device or devices while portions of thelist/database may be locally-cached on other network devices. Exampleswill be illustrated below in the context of leaf-spine fabric overlaynetworks. Thus, depending on which network device has access to therelevant destination host identification-location information—e.g., aparticular entry in the aforementioned list—the destination host'slocation on the network may be determined before or after the packet isforwarded from the first initially-encountered network device receivingthe packet. For example, if the relevant destination host information isaccessible from another network device, the packet may be forwarded tothis second network device and, after said forwarding, the destinationhost's location on the network may be determined at this second networkdevice.

In some embodiments, the IP network which implements the disclosedIP-based packet forwarding techniques may be a leaf-spine networkfabric. Accordingly, presented below and provided in U.S. ProvisionalPat. App. No. 61/900,228, filed Nov. 5, 2013, and titled “NETWORK FABRICOVERLAY” (incorporated by reference in its entirety and for allpurposes) are detailed descriptions of leaf-spine fabric overlaynetworks which, according to this disclosure, may employ mechanisms forforwarding incoming packets to destination hosts based on thedestination IP addresses designated in the incoming packets, and in someembodiments, without reference to the destination MAC address designatedin the incoming packets. Thus, for example, in the case of an ARPrequest packet, although in a legacy layer 2 network an ARP requestpacket is broadcast to all end devices on a local subnet, in variousembodiments of the leaf-spine fabric overlay network set forth below andin U.S. Provisional Pat. App. No. 61/900,228, because an ARP requestpacket includes the intended destination host's IP address, and becausenetwork devices within the leaf-spine network fabric are aware of thelocations where hosts are connected to the network, these networkdevices may forward ARP request packets to their intended destinationhosts without broadcasting the ARP request packets within the fabric. Amapping database may keep the relevant location-identificationinformation concerning the connection of end hosts to the leaf-spinenetwork, in some embodiments, in the form of a list which matches one ormore host IP addresses with one or more locations on the leaf-spinenetwork where the hosts are connected.

Thus, in the context of the leaf-spine fabric overlay networks describedbelow and in U.S. Provisional Pat. App. No. 61/900,228, and referringagain to FIG. 2, various IP-based packet forwarding methods 200 mayinclude an operation 210 of receiving a packet from a source hostconnected to the IP network where the receiving is performed by a leafnetwork device in the fabric which serves as the initial network deviceencountered by the packet when it reaches the leaf-spine fabric overlaynetwork after the packet issues from the source host. After receipt ofthe packet by the leaf network device, methods proceed as shown in FIG.2 with operation 220 of identifying the IP address of a destination hostdesignated in the packet, and then by operation 230 of determining thelocation on the IP network where the destination host designated by thepacket is connected, without reference to the MAC address specified inthe packet. In the context of leaf-spine fabric overlay networks, thelocation determined in operation 230 would typically be a leaf networkdevice within the fabric of the leaf-spine network, although it couldalso be, for example, be a virtual switch device running within theoverlay network's virtualization layer. After the location of thedesignated destination host is determined (by usinglocation-identification information stored on the leaf-spine network),the packet is forwarded in operation 240 to the location within theleaf-spine IP network fabric where the destination host is connected,once again, the forwarding to said location done without reference tothe MAC address specified in the packet. Typically (though notalways—see below), in a leaf-spine fabric overlay network, the locationwhere the destination host connects is a leaf network device whichtherefore serves as the packets egress point from the network.

It is noted that the IP-based forwarding techniques and operationsdisclosed herein may be used in connection with IP networks whichprovide a data abstraction layer oftentimes referred to as an overlaywherein packets are encapsulated with a packet encapsulationscheme/protocol such as VXLAN upon ingress to the network, and arede-encapsulated upon egress from the network. Examples of overlaynetworks in the context of leaf-spine network architectures utilizing aVXLAN encapsulation scheme/protocol are described in U.S. ProvisionalPat. App. No. 61/900,228. Thus, in some embodiments, methods of IP-basedpacket forwarding may include applying an encapsulation to a packetafter being received by the initial network device encountered by thepacket as it reaches the network, and removing the encapsulation fromthe packet as it exits the IP network before it reaches the destinationhost. In the context of a leaf-spine fabric overlay network, theinitially encountered network device is typically a leaf network deviceand so the encapsulation may be applied by this initially encounteredleaf network device. However, it should be noted, or course, thatIP-based packet forwarding techniques and operations do not require theexistence of an overlay network in order to function and provide thebenefits described above.

It should also be noted, particularly in the context of overlaynetworks, that in some embodiments, the location where the destinationhost connects may be a virtual switch device operating in avirtualization layer (running on an underlying physical host) andmoreover that the destination host itself may be a virtual machineoperating in the virtualization layer. (Note that virtualization in thecontext of a leaf-spine fabric overlay network is also described indetail in U.S. Provisional Pat. App. No. 61/900,228.) Likewise, incertain embodiments, the source host which issued the IP packet may be aphysical host connected to a leaf network device which—as the initialnetwork device encountered by the packet when it reaches the leaf-spinefabric overlay network—receives the packet and serves as the packet'singress point to the network. And, likewise, in some embodiments, thesource host may be a virtual machine operating in a virtualization layer(running on an underlying physical host), and the first network “device”in the fabric overlay network encountered by a packet after being issuedfrom the source host may be a virtual switch device also running in thevirtualization layer, which then serves as the packet's ingress point tothe network.

Returning to the manner in which various IP-based packet forwardingmethodologies' may access and utilize location-identificationinformation: In some embodiments, the mapping database containing thelocation-identification information used for determining destinationhost location—e.g., a list matching host IP addresses with networklocations—is associated with the leaf network devices, the spine networkdevices, with both types of devices, or with a third type of devicewhich provides this information with respect to packets forwarded from aleaf or spine network device, or in some combination of the foregoing.

In certain such embodiments, a partial mapping database is associatedwith each leaf network device which may be a locally-cached subset of afull global location-identification mapping database associated with thespine network devices—in some embodiments, stored directly on each spinenetwork device, and in other embodiments stored on a third type ofnetwork device which is associated with the spine network devices.Portions of the spine's global mapping database—which typically liststhe location-identification information associated with every hostconnected to the network through each leaf network device—may be learnedby the leaf network devices as the network operates, as described inU.S. Provisional Pat. App. No. 61/900,228 (incorporated by referenceherein).

Thus, various embodiments of the IP-based forwarding techniques andoperations disclosed herein work (in the ARP context or in the moregeneral IP-based forwarding context) by looking-up an inbound packet'sdestination IP address in a mapping database associated locally with theleaf network device which receives the inbound packet. In suchembodiments, the destination host's location on the network isdetermined at the initially encountered leaf network device before thepacket is first forwarded from the initially encountered leaf networkdevice. In other embodiments, the mapping database may be associatedwith a spine network device and therefore the destination host'slocation on the network is determined from a global mapping databaseassociated with the spine network device after forwarding the packetfrom the leaf network device to a spine network device having access tothis global mapping database. In yet other embodiments, the list may beassociated with another type of network device—a proxy-function networkdevice—which is associated with the spine network device receiving thepacket, but which is used to perform the actual lookup/determination ofthe location of the correct destination host. In certain embodimentswhere packets are encapsulated upon ingress to the IP network, theencapsulation header (e.g., VXLAN header) carries a proxy addressassociated with or designating this proxy-function network device. Theproxy-address may be carried in the destination address field of theencapsulation header, and after the packet is received at theproxy-function network device, said device may replace the proxy-addresswith the actual location/address on the network where the destinationhost connects. As mentioned above, whether the determination ofdestination host location is done at the initially encountered leafnetwork device or at a spine-network device (or proxy-function networkdevice) after being forwarded from this leaf network device may dependon whether the destination host's location is present in the leafnetwork device's locally cached subset of the global mapping databaseassociated with the spine. In any event, mapping database(s) which havethe relevant location-identification information are employed in theforegoing manner to determine the location within an IP network where agiven destination host is located and connected.

To further facilitate an understanding of mapping database usage inIP-based forwarding operations performed in the context of leaf-spinenetwork architectures, a brief description of these architectures is nowprovided. A more detailed description is provided further below.

FIGS. 4 and 5 collectively present a comparison between a traditional“access-aggregation-core” (ACC) network 400 and a simple leaf-spinenetwork 500. Both networks consist of 12 end hosts connected togethervia 8 network devices. In particular, FIG. 4 schematically illustratesan ACC network of 12 end devices/hosts 410 (e.g., servers) connectedthrough an access tier 420, an aggregation tier 430, and a top-levelcore tier 440. The network devices 425 within each tier (e.g., ethernetswitches) control the flow of network traffic over the various links 415(e.g., ethernet cable) between it and the other network devices 425, andultimately to and from end devices/hosts 110. As shown in FIG. 4, it isaccess tier 420 which provides each end device 410 with immediate accessto the network. From there, traffic may pass to the aggregation tier430, and then to the core tier 440, depending on its final destination.

The basic leaf-spine network 500 presented in FIG. 5 resembles (to acertain extent) the AAC network 400 of FIG. 4. As with ACC network 400,leaf-spine network 500 provides connectivity for 12 end devices whichdirectly connect to 5 network devices in a first tier. In the case ofleaf-spine network 500, the 12 hosts connect to the 5 leaf networkdevices 525 of leaf tier 520, which are analogous to the 5 networkdevices 425 of the access tier 420 of the AAC network 400. However,moving on to the second tier, unlike the AAC network 400 which has a2-network device aggregation tier 430 and a 1-network device core tier440, the leaf-spine network 500 employs just a single additional tier,the spine tier 530, which consists of 3 spine-network devices 535. Adetailed comparison of these two network architectures is presentedbelow. In addition, FIGS. 6A-1, 6A-2, and 6B described belowschematically illustrate the formation of leaf-spine networks havingmore than two tiers. Here, it is sufficient to generally note that abenefit of the leaf-spine architecture is that it is less hierarchicalthan its traditional ACC counterpart. For instance, in ACC network 400,depending on source and destination end hosts 410 involved, traffic maybe routed through the single network device 425 in core tier 440,whereas in the leaf-spine network 500, traffic may be routed between anycombination of end hosts 510 connected to leaf network devices 525through several different spine network devices 535 in the spine tier530.

FIG. 7 schematically illustrates the operation of an IP-based packetforwarding technique in the context of a simple leaf-spine network 700having 3 leaf network devices 721, 722, 723 and 2 spine network devices731, 732 which collectively connect together 4 end hosts 711, 712, 713,714. Note that for simplicity and unlike the preceding figures, themultitude of connections between the network devices are not drawn in,so as to not obscure the description of various paths through the fabricwhich follows below. Also shown in the figure as a component of network700 is proxy-function network device 741. Note that while proxy-functionnetwork device 741 is shown communicating with spine network device 732,depending on the embodiment, it may also be used to determine thenetwork locations of end hosts designated by packets routed throughspine network device 731; or spine network device 731 may utilize aseparate dedicated proxy-function network device.

Thus, among other things, FIG. 7 presents an IP network having a firstset of multiple network devices (leaf network devices 721, 722, and 223)for connecting multiple hosts (711, 712, 713, 714) to the network, and asecond set of multiple network devices (spine network devices 731, and732) for connecting together the first set of network devices, and athird type/set of network device(s) (in this example, just one device,proxy-function network device 741) which in certain circumstances(described below) may provide the locations of destination hosts on thenetwork. The particular embodiment schematically illustrated in FIG. 7involves a leaf-spine architecture having local mapping databasesassociated with each leaf network device 721, 722, 723 each of whichrepresents a locally-cached partial copy of a global mapping databaseassociated with spine network devices 731, 732, utilized by the spinethrough proxy-function network device 741. Three paths through thefabric are illustrated in FIG. 7, 751 (dotted line), 752 (dotted-dashedline), and 753 (dashed line)—each of which represent the communicationof a packet from source host 711 to one of three destination hosts, 712,713, and 714, respectively.

In a typical embodiment, each leaf network device's locally-cachedpartial mapping database will contain entries for the end hosts directlyconnected to it. Hence, communication between end hosts 711 and 712,which are both directly connected to leaf network device 721, may beaccomplished without involving the spine, as illustrated by path 751labeled ‘local’ in FIG. 7. Thus, IP-based forwarding of this particularpacket is accomplished with a local mapping database (e.g., list oflocation-identification information) associated with this initiallyencountered leaf network device 721, which is used to determinedestination host 712's location on the network (e.g., its directconnection to leaf network device 721) so that the packet may beforwarded directly from leaf network device 721 to end host 712. Innetwork architectures employing packet encapsulation (e.g., VXLAN),because packet forwarding between source and destination hosts connectedto the same leaf network device only involves local forwarding and notcommunication through the fabric of the network to the spine, this localforwarding may be done without using packet encapsulation. Although anencapsulation could still be done, the encapsulation would be applied byleaf network device 721, for example, upon receipt of a packet from host711, and then immediately removed before forwarding the packet todestination host 712. In other words, the encapsulated packet would notbe transmitted, making the encapsulation effectively superfluous.

Path 752 shown in FIG. 7 schematically illustrates another packetcommunication through the fabric of leaf-spine network 700, this timebetween end hosts 711 and 713 which are not connected to the same leafnetwork device. As shown by path 752, in this instance, the packet doesget routed through the spine—specifically spine network device 731.However, as indicated by path 752's label in the figure, although thecommunication of this packet is non-local to leaf network device 721, anentry for end host 713 was present in leaf network device 721'slocally-cached mapping database, and so end host 713's location on thenetwork (e.g., its connection to leaf network device 722) was determinedprior to the packet leaving leaf network device 721, and withoutinvolving the proxy-function associated with the spine. Accordingly, aswith the previous packet-forwarding instance, forwarding is accomplishedthrough use of the local mapping database of the initially encounteredleaf network device, which is used to determine the destination host'slocation before the packet is forwarded from the initially encounteredleaf network device. In network architectures employing an encapsulationscheme (e.g., VXLAN), here the packet would be encapsulated by leafnetwork device 721 with the destination address field of theencapsulation header set to the address of leaf network device 722, theleaf network device connecting destination host 713 to the network.

Thus, FIG. 7 illustrates that in some embodiments, an IP network mayinclude a first set of network devices (e.g., leaf network devices 721,722, and 223) which have logic (e.g., implemented in hardware and/orsoftware) for receiving a packet from a connected source host (e.g.,711, 712, 713, 714), logic for identifying the IP address of adestination host designated in a received packet, logic for attemptingto determine the location on the network where a destination hostdesignated by a received packet is connected (without reference to theMAC address specified in the received packet), logic for labeling thepacket with this location, and logic for forwarding the packet to anetwork device in a second set of network devices (e.g., spine networkdevices 731, 732) when the determined location not local to networkdevice having received the packet. Of course, FIG. 7 also illustratesthat in some embodiments, an IP network may include this second set ofnetwork devices (e.g., spine network devices 731, and 732) each of whichhave logic for receiving packets from the network devices in the firstset. Generally, the network devices in the first and second sets mayemploy logic (e.g., implemented in hardware and/or software) forperforming any, many, or all of the operations appropriate for theirfunction as discussed herein. Thus, for instance, the network devices ofthe first set may include logic for applying an encapsulation (e.g., aVXLAN encapsulation) before forwarding a packet to the spine, and logicfor removing the encapsulation before forwarding the packet to end hostfrom egress leaf network device, as described above.

Another packet's passage through the fabric is illustrated by path 753,which represents a communication between end hosts 711 and 714. In thisinstance, as with path 752, the communication between end hosts isnon-local and involves multiple leaf network devices but, as indicatedby the path 753's label in FIG. 7, the location-identificationinformation associated with end host 714 is not found in thelocally-cached mapping database associated with leaf network device 711.Hence, leaf network device 721 labels this packet with the location ofthe proxy-function network device 741—e.g., sets the destination addressfield of the encapsulation header to the proxy address in networksemploying an encapsulation scheme (e.g., VXLAN)—and after this packet isforwarded to the spine—in this instance, to spine network device 732—thepacket is forwarded to proxy-function network device 741. Theproxy-function network device 721 then determines end host 714'slocation on the network (e.g., its connection to leaf network device723) by looking up end host 714 in the global mapping database andreplaces the contents of the packet header field designating theproxy-function location—e.g., the destination address field of theencapsulation header—with that of end host 714's network location. Thus,in this IP-based packet-forwarding embodiment, forwarding isaccomplished utilizing a mapping database associated with spine networkdevice 732 (via proxy-function network device 741) which is used todetermine destination host 714's location on the network after thepacket is initially forwarded from leaf network device 721 to the spine.In this embodiment, logic for identifying the IP address of thedestination host designated in the received packet resides on theproxy-function network device 741, however in other embodiments, saidlogic may reside on the spine network device itself.

Thus, in some network architecture embodiments, iflocation-identification information corresponding to the destination IPaddress designated in an inbound packet is found in the local mappingdatabase associated with the initial network device receiving theinbound packet, the packet will be forwarded accordingly—e.g., if thedestination host is local to the leaf network device receiving thepacket, the packet will be forwarded out a local port on the leafnetwork device to the destination host. However, if the destination hostis remote from the ingress leaf network device, the packet will beencapsulated (e.g. with VXLAN), the encapsulation carrying the addressof the remote leaf network device to which the destination host isconnected, and sent towards an appropriate spine network device. In someembodiments, if there is a miss in the local mapping database (cache oflocation-identification information), the packet will be encapsulatedwith the proxy IP address and sent towards a spine network device thathas the proxy function or is associated with a third type of networkdevice providing the proxy function. The proxy function then operates todetermine the location of the host on the network having the destinationIP address designated in the received packet.

In any event, referring again to FIG. 7 and specifically to paths 752and 753, once the location of the designated end host is determined, thespine network device having received the packet forwards the packet(whether an ARP request packet or an IP packet, generally) to the leafnetwork device which is the location on the IP network where thedesignated destination host is connected/attached. For the instance ofpath 752, the packet is forwarded from spine network device 731 to leafnetwork device 722, and likewise for path 753, the packet is forwardedfrom spine network device 732 to leaf network device 723 (after spinenetwork device 732 receives the packet back from proxy-function networkdevice 741). Finally, paths 752 and 753 illustrate their correspondingpackets being forwarded from leaf network devices 722 and 723 to endhosts 713 and 714, respectively. In a network employing encapsulation,the encapsulation would typically be removed from the packet at thispoint—before forwarding from egress leaf network device to end host.However, in embodiments where the end host is a virtual server/machinerunning within a virtualization layer atop a physical host/server (e.g.,running in a “hypervisor”), a virtual switch device (V-switch) withinthe virtualization layer may represent the packet's egress point fromthe overlay network and also the location on the network where thevirtual host connects. Accordingly, in such an embodiment, packets wouldbe un-encapsulated by this V-switch device. Once again, virtualizationtechnology within the context of leaf-spine fabric overlay networks isdescribed in detail in U.S. Provisional Pat. App. No. 61/900,228(incorporated by reference herein). In embodiments where the designateddestination host is a physical host/server, Layer 2 communicationprotocols may be used to complete the packet's final hop from the edgeof the leaf-spine network fabric to the connected host. Thus, in someembodiments, after the packet is de-encapsulated, it may be labeled withthe MAC address of the destination host, prior to forwarding to thedestination host. The egress leaf network device would typically havethis MAC address in its local forwarding table, along with the MACaddresses of any other physical hosts it connects to the leaf-spinefabric overlay network.

As indicated above, the foregoing IP-based packet forwarding techniquesand operations may be used to handle ARP request packets and preventtheir broadcast (generation of broadcast loops, etc.) within the fabricof a leaf-spine network while preserving the external semanticalbehavior expected by hosts connected via Layer 2 to the network. In oneembodiment, an ARP request packet may be forwarded via the IP-basedforwarding techniques described above to the leaf network device whichconnects the end host having the IP address designated in the ARPrequest packet. At this point, in networks employing packetencapsulation, this leaf network device—since it serves as the ARPrequest packet's egress point from the network—may de-encapsulate theARP request packet prior to forwarding the packet to target destinationhost designated in the packet. Note that if more than one host isconnected on this interface of the leaf network device—the interfaceconnecting the destination host—forwarding of the ARP packet out thisinterface effectively broadcasts the ARP packet out this interface sincethe ARP packet is now un-encapsulated and it's destination MAC addressfield is still labeled “broadcast” as shown in FIG. 1A (the same as itwas upon ingress to the network through the initial leaf networkdevice). In any event, the un-encapsulated ARP request packet reachesthe destination host, and that host then prepares an ARP reply packethaving its source MAC address field populated with the destinationhost's MAC address as shown in FIG. 1B. Note that the entire techniquepreserves, external to the network, Layer 2 ARP semantics as far assource and destination hosts are concerned. One advantage is that thisprovides the destination host an opportunity to respond to the ARPrequest packet, thereby allowing the whole ARP-ing procedure to serve asa “live-ness” check on the destination host by the source host. However,in other embodiments, the leaf network device which connects thedestination host to the network may itself prepare an ARP responsepacket in reply to the ARP request packet, without notifying theattached target device, saving a potential forwarding step, but at theexpense of the “live-ness” check. This may be done using a locallystored MAC address for the destination host present in a forwardingtable associated with the leaf network device which identifies the MACaddresses of each attached/connected host and/or other device.

Accordingly, in some embodiments, a leaf network device in a leaf-spinenetwork fabric may receive an ARP request packet from one of itsattached hosts or other external devices which is labeled for broadcast.However, the leaf device prevents the packet's broadcasting byforwarding the packet based on the “target IP” address found in thepayload of the packet, rather than in the conventional way by forwardingthe packet based on the Layer 2 destination address, which is abroadcast address. To provide a specific, non-limiting example: host Aconnected to a leaf-spine fabric overlay network wants to communicatewith host B also connected to the network, but host A does not know hostB's MAC address. Host A therefore generates an ARP request packet andforwards it onto the network. The first network device receiving the ARPrequest packet is the leaf network device to which host A is attached.The ARP request packet includes the following information similarly tothat shown in FIG. 1A:

-   -   L2 Destination MAC Address=Broadcast    -   L2 Source MAC address=Host A's MAC address    -   Payload contains: Host B's IP address

The ingress leaf network device analyzes this ARP request packet andidentifies Host B's IP address in the packet's payload. If this leafnetwork device determines from Host B's IP address that host B islocally connected to itself, this leaf network device forwards thepacket directly to host B without encapsulating it. If the ingress leafnetwork device recognizes host B's IP address, but determines that itisn't a local IP address, this leaf network device encapsulates thepacket and forwards it to the spine, the encapsulation identifying theIP address of the leaf network device connecting host B. If the ingressleaf network device does not recognize host B's IP address, this leafnetwork device produces an encapsulation identifying the IP address of anetwork device providing the proxy function as the destination IPaddress—either a spine network device or another class of network devicewhich provides the proxy function—and forwards the packet to thespine—which then either applies the proxy function or forwards thepacket to a proxy-function network device which applies the proxyfunction and forwards the packet back to the spine. With the packet'sencapsulation now identifying the leaf network device connecting host B,the spine network device then sends the ARP request packet to this leafnetwork device. Note, once again, that the same forwarding proceduregenerally applies to other types of packets which specify a destinationIP address.

In this example of an ARP request packet going from host A to host B,after forwarding from the spine, the receiving leaf network devicerecognizes the packet as an ARP request and recognizes host B's IPaddress. The receiving leaf network device may optionally update itsforwarding table with information about host A. The leaf network devicethen forwards the packet to host B, which prepares and sends an ARPreply packet back to the leaf network device. The leaf network devicenow receives and forwards this ARP reply packet to the spine, which thenroutes the ARP reply to the leaf network device locally connecting hostA. That leaf network device then de-encapsulates the ARP reply andforwards the ARP reply to host A. At this point, the leaf network deviceconnecting host A may update its own forwarding table with informationabout host B.

Note that the gathering of the location-identification information forthe mapping database cached at the leaf network devices may be donethrough protocols or through learning of the devices attached to thenetwork, for example, as demonstrated in the preceding ARP example. Thelocation-identification information in a local mapping database mayinclude MAC and IP addresses of most or all locally connected hostdevices, however, as described above, these local mapping databases neednot contain the MAC addresses of every host connected to every leafnetwork device on the network. In some embodiments as described above,the learned location-identification information may be provided in amapping database resident on the spine, portions of which arelocally-cached in the leaf network devices. Of course, it should also benoted that IP-based packet forwarding—whether applied to IP packetsgenerally, or in the context of unicast ARP—may be implemented withoutan overlay network, and also in networks having other topologies besidesthe leaf-spine fabric now described in detail.

Detailed Description of Leaf-Spine Network Architectures VersusTraditional Network Architectures

A. Overview of Traditional “Access-Aggregation-Core” NetworkArchitectures

Datacenter network design may follow a variety of topologicalparadigms—a given topology just referring to the system of networkinglines/links which carry network traffic (i.e., data) and the networkingswitches, which control the flow of traffic over the lines/links in thenetwork. One of the most common topological paradigms in use today isthe aptly-named “access-aggregation-core” architecture. As the “core”part of the name suggests, such an architecture follows a hierarchicalparadigm, wherein information traveling between hypothetical points Aand B, first travel up the hierarchy away from point A and then backdown the hierarchy towards point B.

FIG. 4 schematically illustrates a simple network 400 made up of 12 “enddevices” 410 (e.g., servers) which, following the“access-aggregation-core” (AAC) model, are connected through an accesstier 420, an aggregation tier 430, and a top-level core tier 440. Withineach tier are “network devices” 425 (e.g., ethernet switches) each ofwhich controls flow of network traffic over various “links” 415 (e.g.,ethernet cable) between it and the other network devices 425 andultimately to and from end devices 110. As shown in FIG. 4, it is accesstier 420 which provides each end device 410 with immediate access to thenetwork. From there, traffic may pass to the aggregation tier 430, andthen to the core tier 440, depending on its final destination. It isnoted that for traffic whose final destination is within the network 400shown in FIG. 4, how far up the hierarchy the traffic must be directedto reach this destination depends on the location of the destinationwithin the network. Traffic whose final destination is outside thenetwork shown in FIG. 4—e.g., to some server on the worldwideintenet—will typically travel all the way up the hierarchy, since theconnection or connections to the outside internet typically reside inthe core tier. It is also noted that while FIG. 4 portrays each networkdevice 425 identically for purposes of illustration, actual hardwareimplementations of such devices may possess substantial differencesdepending on whether the device is located in the access tier 420, theaggregation tier 430, or the core tier 440. For one thing, the singlenetwork device in the core tier 440 of FIG. 4 would typically route moretraffic (for the reasons just described) than those network devices inthe other layers, and so it's hardware would be chosen accordingly.Moreover, differing hardware implementations in one tier versus anothermay be dictated by whether the devices in each tier do bridging orrouting, or both. For instance, the access tier typically does onlybridging, the aggregation tier a combination of bridging androuting—bridging and routing between interfaces going to the access tierand routing to all others—and the core tier, only routing. Note that theterm “end device” 410 refers to a device connected to the network whichis identified on the network and uses the network to communicate withother devices connected to the network. As such, the end device 410 maybe personal computer, a workstation, a server, or a device which may notbe a computer per se such as, for example, a network printer, scanner,network storage device, etc. Also note that while a computer, server,workstation, etc. may be a physical device having one or more processors(each potentially having one or more cores) connected to an array ofrandom-access memory (RAM) and possibly a non-volatile storage medium(such as a magnetic disc, flash drive, optical disc, magnetic tape,etc.), an end device 410 may also be a virtual device, such as avirtualized server running in a virtualization environment—such asVMWare—atop an actual physical server. Finally, it is also noted that insome embodiments, what is designated as an “end device,” may actuallyrefer to multiple computers, the distinction being that for purposes ofnetwork topology they share the same connection to the network andtherefore operate substantially as a single networked unit.

FIG. 4 illustrates the hierarchal connectivity of aaccess-aggregation-core (AAC) network model and shows that, from core toend device, the architecture follows a tree structure or graph—meaningthat each network device within the network is only connected to asingle device above it in the hierarchy, but is potentially connected tomultiple network devices below it. The tree structure also implies, asseen from the figure, that there is a single unique path—i.e., sequenceof links—connecting any two end devices on the network. However, thougheach pair of end devices is connected by a unique path, paths betweendifferent pairs of end devices may, but not necessarily, use the samelinks along some portion of their route. For example, with regards tothe specific network shown in FIG. 4, the end devices 410 labeled A andF (hereinafter “end device A” and so forth) communicate over a paththrough network devices 1, II, and then 3. Likewise end devices I and Jcommunicate over a path through network devices 4, III, and then 5, andso these two paths do not overlap (i.e. the paths have no links incommon). However, if end device A instead communicates with end device K(while I communicates with J), then both paths pass through networkdevices III and 5 (and the link adjoining devices III and 5).

Shared usage of links and network devices (such as just described) leadsto bottlenecks in a network exhibiting a tree structure architecturelike the access-aggregation-core (AAC) network shown in FIG. 4. For sakeof simplicity, assume that in a given AAC network, data transmissionover each link and through each network device is limited to the samemaximum data transmission rate, and that each end device on this networkcan send and receive data at this maximum rate as well. In other words,the same bandwidth limitation applies to the whole network. If this isso, then referring again to FIG. 4, end devices A and B are able tocommunicate at full bandwidth, while end devices I and J are alsocommunicating at full bandwidth. However, to use the previous example,if end device A attempts to communicate with end device K while enddevice I is communicating with end device J, then “blocking”occurs—e.g., either both pairs of end devices communicate at halfmaximum bandwidth, or they communicate at full bandwidth sequentially,one pair waiting for the other pair to finish. Obviously, the situationbecomes much worse in a AAC network if many end devices which aretopologically-separate from each other (i.e., their communication pathsinvolve many links) attempt to simultaneously communicate with oneanother at full bandwidth.

Though the blocking problem is an inevitable consequence of thetree-structure paradigm, various solutions have been developed withinthis paradigm to lessen the impact of the problem. One technique is tobuild redundancy into the network by adding additional links betweenhigh traffic nodes in the network. In reference to FIG. 4, this mightmean adding extra links between the core tier network device I, and theaggregation tier network devices II and III. Split-etherchannel and thelike are examples implementing such an approach. Alternatively, insteadof adding more links, standard-bandwidth links may be replaced byhigher-bandwidth links, but the effect is essentially the same, albeitthe consequences of link failure will be more severe versus havingredundant links. With regards to link failure, it is noted in referenceto FIG. 4 that even if redundant links are employed between the core andaggregation tiers, 1 link failure out of 4 in this simple example wouldreduce the network's bisectional bandwidth by a factor of 2.(“Bisectional bandwidth” is discussed in detail below.) Thus, it isevident that even with built-in redundancy, a tree-structured networkarchitecture has inherent limitations, and, of course, there are costsassociated with implementing hardware-based redundancy.

B. “Leaf-Spine” Network Architectures

Another way of addressing the ubiquitous “blocking” problem manifestedin the modern datacenter's networking infrastructure is to design a newnetwork around a topological paradigm where blocking does not present asmuch of an inherent problem. One such topology is often referred to as a“multi-rooted tree” topology (as opposed to a “tree”), which can be saidto embody a full bi-partite graph if each spine network device isconnected to each Leaf network device and vice versa. Networks based onthis topology are oftentimes referred to as “Clos Networks,” “flatnetworks,” “multi-rooted networks,” or just as “multi-rooted trees.” Inthe disclosure that follows, a “leaf-spine” network architecturedesigned around the concept of a “multi-rooted tree” topology will bedescribed. While it is true that real-world networks are unlikely tocompletely eliminate the “blocking” problem, the described “leaf-spine”network architecture, as well as others based on “multi-rooted tree”topologies, are designed so that blocking does not occur to the sameextent as in traditional network architectures.

Roughly speaking, leaf-spine networks lessen the blocking problemexperienced by traditional networks by being less hierarchical and,moreover, by including considerable active path redundancy. In analogyto microprocessor design where increased performance is realized throughmulti-core or multi-processor parallelization rather than simply byincreasing processor clock speed, a leaf-spine network realizes higherperformance, at least to a certain extent, by building the network “out”instead of building it “up” in a hierarchical fashion. Thus, aleaf-spine network in its basic form consists of two-tiers, a spine tierand leaf tier. Network devices within the leaf tier—i.e. “leaf networkdevices”—provide connections to all the end devices, and network deviceswithin the spine tier—i.e., “spine network devices”—provide connectionsamong the leaf network devices. Note that in a prototypical leaf-spinenetwork, leaf network devices do not directly communicate with eachother, and the same is true of spine network devices. Moreover, incontrast to an AAC network, a leaf-spine network in its basic form hasno third core tier connecting the network devices within the second tierto a much smaller number of core network device(s), typically configuredin a redundant fashion, which then connect to the outside internet.Instead, the third tier core is absent and connection to the internet isprovided through one of the leaf network devices, again effectivelymaking the network less hierarchical. Notably, internet connectivitythrough a leaf network device avoids forming a traffic hotspot on thespine which would tend to bog down traffic not travelling to and fromthe outside internet.

It should be noted that very large leaf-spine networks may actually beformed from 3 tiers of network devices. As described in more detailbelow, in these configurations, the third tier may function as a “spine”which connects “leaves” formed from first and second tier networkdevices, but a 3-tier leaf-spine network still works very differentlythan a traditional AAC network due to the fact that it maintains themulti-rooted tree topology as well as other features. To present asimple example, the top tier of a 3-tier leaf-spine network still doesnot directly provide the internet connection(s), that still beingprovided through a leaf network device, as in a basic 2-tier leaf-spinenetwork.

FIG. 5 schematically illustrates a particular example of a basicleaf-spine network 500. To a certain extent, network 500 is analogous(or is the counterpart of) the AAC network 400 shown in FIG. 4. Bothprovide connectivity for 12 end devices which directly connect to 5network devices in a first tier, in the case of network 500, to the 5leaf network devices 525 of leaf tier 520, which are analogous to the 5network devices 425 of the access tier 420 of the AAC network 400.However, moving on to the second tier, unlike the AAC network 400 whichhas a 2-network device aggregation tier 430 and a 1-network device coretier 440, the leaf-spine network 500 employs just a single additionaltier, the spine tier 530, which consists of 3 spine-network devices 535.

Though in FIGS. 4 and 5 the total number of network devices in thehigher tiers remains the same, the connectivity of these devices isquite different. FIG. 5 shows that in a prototypical leaf-spine network,every leaf network device 525 is connected to multiple spine networkdevices 535 creating the so-called “multi-rooted tree”topology—differing from the ordinary tree topology of an AAC networkwhere each network device is connected to only one network device aboveit in the hierarchy. As a consequence and unlike an AAC network, in aleaf-spine network there are multiple paths of communication between anypair of leaf network devices 525, and thus also between any pair of enddevices 510 connected to different leaf network devices. The multi-pathtopology which joins the end devices in these networks is acharacteristic which helps to reduce the extent to which “blocking”poses a problem. Moreover, leaf-spine networks are such that if asufficient number of spine network devices are connected with sufficientbandwidth to the leaf network devices, a leaf-spine network may providewhat is referred to as “full bisectional bandwidth,” as described inmore detail below. Furthermore, by adding additional tiers (such as athird tier as mentioned above and as described in greater detail below),a network of arbitrary size can be built that still provides “fullbisectional bandwidth.”

To illustrate, consider analogously to the example described above,communication between end device A and end device K simultaneous withcommunication between end devices I and J, which led to blocking in AACnetwork 400. As shown in FIG. 5, in the leaf-spine network 500, thereare three paths connecting end device A with end device K and threedifferent paths connecting end device I with end device J illustratingthat no blocking problem arises. Again, assuming all links in thenetwork provide equal bandwidth, the two pairs of end devices each haveavailable 3 times the required bandwidth for max rate communication andthus the network is non-blocking in this scenario.

As a second example, consider the scenario of simultaneous communicationbetween end devices A and F and between end devices B and G which willclearly also lead to blocking in AAC network 400. In the leaf-spinenetwork 500, although two leaf network devices 525 are shared betweenthe four end devices 510, specifically network devices 1 and 3, thereare still three paths of communication between these two devices (onethrough each of the three spine network devices I, II, and III) andtherefore there are three paths collectively available to the two pairsof end devices. Thus, it is seen that this scenario is also non-blocking(unlike FIG. 4) because each pair of end devices still has access to a1.5 times the bandwidth required for max rate communication.

As a third example, consider the scenario of simultaneous communicationbetween three pairs of end devices—between A and F, between B and G, andbetween C and H. In AAC network 400, this results in each pair of enddevices having ⅓ the bandwidth required for full rate communication, butin leaf-spine network 500, once again, since 3 paths are available, eachpair has exactly the bandwidth it needs for full rate communication.Thus, in a leaf-spine network having single links of equal bandwidthconnecting devices, as long as the number of spine network devices 535is equal to or greater than the number of end devices 510 which may beconnected to any single leaf network device 525, then the network willhave enough bandwidth for simultaneous full-rate communication betweenthe end devices connected to the network.

More generally, the extent to which a given network is non-blocking maybe characterized by the network's “bisectional bandwidth,” which isdetermined by dividing a network that has N end devices attached to itinto 2 equal sized groups of size N/2, and determining the totalbandwidth available for communication between the two groups. If this isdone for all possible divisions into groups of size N/2, the minimumbandwidth over all such divisions is the “bisectional bandwidth” of thenetwork. Based on this definition, a network may then be said to have“full bisectional bandwidth” and have the property of being “fullynon-blocking” if each leaf network device's total uplink bandwidth tothe spine tier 530 (the sum of the bandwidths of all links connectingthe leaf network device 525 to any spine network device 535) is at leastequal to the maximum downlink bandwidth to end devices associated withany of the leaf network devices on the network.

To be precise, when a network is said to be “fully non-blocking” itmeans that no “admissible” set of simultaneous communications betweenend devices on the network will block—the admissibility constraintsimply meaning that the non-blocking property only applies to sets ofcommunications that do not direct more network traffic at a particularend device than that end device can accept as a consequence of its ownbandwidth limitations. Whether a set of communications is “admissible”may therefore be characterized as a consequence of each end device's ownbandwidth limitations (assumed here equal to the bandwidth limitation ofeach end device's link to the network), rather than arising from thetopological properties of the network per se. Therefore, subject to theadmissibility constraint, in a non-blocking leaf-spine network, all theend devices on the network may simultaneously communicate with eachother without blocking, so long as each end device's own bandwidthlimitations are not implicated.

The leaf-spine network 500 thus exhibits full bisectional bandwidthbecause each leaf network device has at least as much bandwidth to thespine tier (i.e., summing bandwidth over all links to spine networkdevices) as it does bandwidth to the end devices to which it isconnected (i.e., summing bandwidth over all links to end devices). Toillustrate the non-blocking property of network 500 with respect toadmissible sets of communications, consider that if the 12 end devicesin FIG. 5 are arbitrarily divided into 6 pairs, simultaneouscommunications between the 6 pairs are admissible, and thus may occurwithout blocking in network 500. In addition, it is noted that thenon-blocking property of leaf-spine network 500 will be preserved if upto 15 end devices are connected, 3 to each of the 5 leaf networkdevices.

To implement leaf-spine network 500, the leaf tier 520 would typicallybe formed from 5 ethernet switches of 6 ports or more, and the spinetier 530 from 3 ethernet switches of 5 ports or more. The number of enddevices which may be connected is then the number of leaf tier switchesj multiplied by ½the number of ports n on each leaf tier switch, or½·j·n, which for the network of FIG. 5 is ½·5·6=15. Furthermore, thenumber of ports m on each spine tier switch is equal to the number ofleaf tier switches j (so long as the maximum number of leaf tierswitches are used), and so the total number of end devices is also givenby ½·m·n, where m is the number of ports on the spine tier switches, andn is the number of ports on the leaf tier switches.

However, not every network is required to be non-blocking and, dependingon the purpose for which a particular network is built and the network'santicipated loads, a fully non-blocking network may simply not becost-effective. Nevertheless, leaf-spine networks still provideadvantages over traditional networks, and they can be made morecost-effective, when appropriate, by reducing the number of devices usedin the spine tier, or by reducing the link bandwidth between individualspine and leaf tier devices, or both. In some cases, the cost-savingsassociated with using fewer spine-network devices can be achievedwithout a corresponding reduction in bandwidth between the leaf andspine tiers by using a leaf-to-spine link speed which is greater thanthe link speed between the leaf tier and the end devices. If theleaf-to-spine link speed is chosen to be high enough, a leaf-spinenetwork may still be made to be fully non-blocking—despite saving costsby using fewer spine network devices.

The extent to which a network having fewer spine tier devices isnon-blocking is given by the ratio of bandwidth from leaf network deviceto spine tier versus bandwidth from leaf network device to end devices.By adjusting this ratio, an appropriate balance between cost andperformance can be dialed in. In FIG. 5, for example, assuming linkshave equal bandwidth, one spine network device 535 could be eliminatedif a non-blocking ratio of 2/3 was acceptable. This would imply that if3 end devices connected to a single leaf network device attemptsimultaneous communication to 3 or more end devices attached to otherleaf networking devices, only ⅔ of the required bandwidth would beavailable. This is also referred to as “oversubscription.” In this case,the “oversubscription rate” would be 1.5, since 1.5=(⅔)⁻¹.

This concept of oversubscription and building cost-effective networkshaving fewer than optimal spine network devices also illustrates theimproved failure domain provided by leaf-spine networks versus theirtraditional counterparts. In a traditional AAC network, if a device inthe aggregation tier fails, then every device below it in the network'shierarchy will become inaccessible until the device can be restored tooperation. Furthermore, even if redundancy is built-in to thatparticular device, or if it is paired with a redundant device, or if itis a link to the device which has failed and there are redundant linksin place, such a failure will still result in a 50% reduction inbandwidth, or a doubling of the oversubscription. In contrast,redundancy is intrinsically built into a leaf-spine network and suchredundancy is much more extensive. Thus, as illustrated by theusefulness of purposefully assembling a leaf-spine network with fewerspine network devices than is optimal, absence or failure of a singledevice in the spine (or link to the spine) will only typically reducebandwidth by 1/k where k is the total number of spine network devices.

It is also noted once more that in some networks having fewer than theoptimal number of spine network devices (e.g., less than the number ofend devices connecting to the leaf network devices), theoversubscription rate may still be reduced (or eliminated) by the use ofhigher bandwidth links between the leaf and spine network devicesrelative to those used to connect end devices to the leaf networkdevices.

C. Example “Leaf-Spine” Network Architecture

The following describes a sample implementation of a leaf-spine networkarchitecture. It is to be understood, however, that the specific detailspresented here are for purposes of illustration only, and are not to beviewed in any manner as limiting the concepts disclosed herein. Withthis in mind, leaf-spine networks may be implemented as follows:

Leaf network devices may be implemented as ethernet switches having: (i)48 ports for connecting up to 48 end devices (e.g., servers) at datatransmission speeds of 10 GB/s (gigabits per second)—i.e. ‘downlinkports’; and (ii) 12 ports for connecting to up to 12 spine networkdevices at data transmission speeds of 40 GB/s—i.e. ‘uplink ports.’Thus, each leaf network device has 480 GB/s total bandwidth availablefor server connections and an equivalent 480 GB/s total bandwidthavailable for connections to the spine tier. More generally, leafnetwork devices may be chosen to have a number of ports in the range of10 to 50 ports, or 20 to 100 ports, or 50 to 1000 ports, or 100 to 2000ports, wherein some fraction of the total number of ports are used toconnect end devices (‘downlink ports’) and some fraction are used toconnect to spine network devices (‘uplink ports’). In some embodiments,the ratio of uplink to downlink ports of a leaf network device may be1:1, or 1:2, or 1:4, or the aforementioned ratio may be in the range of1:1 to 1:20, or 1:1 to 1:10, or 1:1 to 1:5, or 1:2 to 1:5. Likewise, theuplink ports for connection to the spine tier may have the samebandwidth as the downlink ports used for end device connection, or theymay have different bandwidths, and in some embodiments, higherbandwidths. For instance, in some embodiments, uplink ports may havebandwidths which are in a range of 1 to 100 times, or 1 to 50 times, or1 to 10 times, or 1 to 5 times, or 2 to 5 times the bandwidth ofdownlink ports.

Moreover, depending on the embodiment, leaf network devices may beswitches having a fixed number of ports, or they may be modular, whereinthe number of ports in a leaf network device may be increased by addingadditional modules. The leaf network device just described having 48 10GB/s downlink ports (for end device connection) and 12 40 GB/s uplinkports (for spine tier connection) may be a fixed-sized switch, and issometimes referred to as a ‘Top-of-Rack’ switch. Fixed-sized switcheshaving a larger number of ports are also possible, however, typicallyranging in size from 50 to 150 ports, or more specifically from 64 to128 ports, and may or may not have additional uplink ports (forcommunication to the spine tier) potentially of higher bandwidth thanthe downlink ports. In modular leaf network devices, the number of portsobviously depends on how many modules are employed. In some embodiments,ports are added via multi-port line cards in similar manner to thatdescribed below with regards to modular spine network devices.

Spine network devices may be implemented as ethernet switches having 576ports for connecting with up to 576 leaf network devices at datatransmission speeds of 40 GB/s. More generally, spine network devicesmay be chosen to have a number of ports for leaf network deviceconnections in the range of 10 to 50 ports, or 20 to 100 ports, or 50 to1000 ports, or 100 to 2000 ports. In some embodiments, ports may beadded to a spine network device in modular fashion. For example, amodule for adding ports to a spine network device may contain a numberof ports in a range of 10 to 50 ports, or 20 to 100 ports. In thismanner, the number of ports in the spine network devices of a growingnetwork may be increased as needed by adding line cards, each providingsome number of ports. Thus, for example, a 36-port spine network devicecould be assembled from a single 36-port line card, a 72-port spinenetwork device from two 36-port line cards, a 108-port spine networkdevice from a trio of 36-port line cards, a 576-port spine networkdevice could be assembled from 16 36-port line cards, and so on.

Links between the spine and leaf tiers may be implemented as 40GB/s-capable ethernet cable (such as appropriate fiber optic cable) orthe like, and server links to the leaf tier may be implemented as 10GB/s-capable ethernet cable or the like. More generally, links, e.g.cables, for connecting spine network devices to leaf network devices mayhave bandwidths which are in a range of 1 GB/s to 1000 GB/s, or 10 GB/sto 100 GB/s, or 20 GB/s to 50 GB/s. Likewise, links, e.g. cables, forconnecting leaf network devices to end devices may have bandwidths whichare in a range of 10 MB/s to 100 GB/s, or 1 GB/s to 50 GB/s, or 5 GB/sto 20 GB/s. In some embodiments, as indicated above, links, e.g. cables,between leaf network devices and spine network devices may have higherbandwidth than links, e.g. cable, between leaf network devices and enddevices. For instance, in some embodiments, links, e.g. cables, forconnecting leaf network devices to spine network devices may havebandwidths which are in a range of 1 to 100 times, or 1 to 50 times, or1 to 10 times, or 1 to 5 times, or 2 to 5 times the bandwidth of links,e.g. cables, used to connect leaf network devices to end devices.

In the particular example of each spine network device implemented as a576-port @ 40 GB/s switch and each leaf network device implemented as a48-port @ 10 GB/s downlink & 12-port @ 40 GB/s uplink switch, thenetwork can have up to 576 leaf network devices each of which canconnect up to 48 servers, and so the leaf-spine network architecture cansupport up to 576·48=27,648 servers. And, in this particular example,due to the maximum leaf-to-spine transmission rate (of 40 GB/s) being 4times that of the maximum leaf-to-server transmission rate (of 10 GB/s),such a network having 12 spine network devices is fully non-blocking andhas full cross-sectional bandwidth.

As described above, the network architect can balance cost withoversubscription by adjusting the number of spine network devices. Inthis example, a setup employing 576-port switches as spine networkdevices may typically employ 4 spine network devices which, in a networkof 576 leaf network devices, corresponds to an oversubscription rate of3:1. Adding a set of 4 more 576-port spine network devices changes theoversubscription rate to 3:2, and so forth.

Datacenters typically consist of servers mounted in racks. Thus, in atypical setup, one leaf network device, such as the ‘Top-of-Rack’ devicedescribed above, can be placed in each rack providing connectivity forup to 48 rack-mounted servers. The total network then may consist of upto 576 of these racks connected via their leaf-network devices to aspine-tier rack containing between 4 and 12 576-port spine tier devices.

D. Leaf-Spine Network Architectures Formed from More than Two Tiers ofNetwork Devices

The two-tier leaf-spine network architecture described above having576-port @ 40 GB/s switches as spine network devices and 48-port @ 10GB/s downlink & 12-port @ 40 GB/s uplink switches as leaf networkdevices can support a network of up to 27,648 servers, and while thismay be adequate for most datacenters, it may not be adequate for all.Even larger networks can be created by employing spine tier devices withmore than 576 ports accompanied by a corresponding increased number ofleaf tier devices. However, another mechanism for assembling a largernetwork is to employ a multi-rooted tree topology built from more thantwo tiers of network devices—e.g., forming the network from 3 tiers ofnetwork devices, or from 4 tiers of network devices, etc.

One simple example of a 3-tier leaf-spine network may be built from just4-port switches and this is schematically illustrated in FIGS. 6A-1,6A-2, and 6B, in order to convey the general principle. Of course, it isto be understood that to build a large 3-tier network, larger switcheswould be employed. FIG. 6A-1 shows 4 4-port switches 622 (labeled “1-1,”“1-2,” “1-3,” “1-4”) connected to form a 2-tier leaf-spine network 601for connecting 4 end devices 610 (labeled “A,” “B,” “C,” “D”) withswitches 1-1 and 1-2 serving as leaf network devices, and switches 1-3and 1-4 serving as spine network devices. Note that a leaf-spine networkassembled from 4-port switches would generally support a network of 8end devices 610, connected through 2 spine-network devices and 4 leafnetwork devices, but here, half the ports on the switches serving asspine network devices, switches 1-3 and 1-4, have their links pointedupward in FIG. 6A-1 to schematically indicate these links are reservedfor uplink connection to a third tier. With the 4 4-port switches 622connected in this manner, they may collectively be viewed as functioningas an 8 port switch 625, as schematically illustrated in FIG. 6A-2 (andlabeled “1”), with 4 downlinks 605 to potential end devices 610 and 4uplinks 615 to a potential third tier. Such an effective 8-port switchmay then serve as a building-block for a 3-tier leaf-spine network.

FIG. 6B then shows how a 3-tier leaf-spine network 602 may be assembledfrom these 8-port building blocks. As shown in the figure, 4 8-portbuilding blocks 625 may effectively serve as 4 leaf-network devices(labeled “1,” “2,” “3,” “4,”) in the leaf tier 620 of network 602. Thespine tier 630 of network 602 is formed from 4 additional 4-portswitches 635 (labeled “I,” “II,” “III,” “IV”), similar to those used toform each 8-port leaf network device 625. Thus, when viewed in terms of4-port switches, network 602 consists of 3 tiers with 8 4-port switchesin the bottom tier, 8 4-port switches in the middle tier, and 4 4-portswitches in the top tier, though this network may still be viewed ashaving a leaf tier 620 and spine tier 630, as just described. It is seenin the figure, that network 602 may connect up to 16 end devices 610.Generalizing this, it is noted that a 3-tier leaf-spine network mayconnect a total number of end devices equal to ¼·l·m·n, where l, m, andn are the number of ports in the devices forming the top, middle, andbottom tiers, respectively (e.g., in FIG. 7B, ¼·4·4·4=16). Thus, ifn-port devices are used to build a network, the size of a 3-tierleaf-spine network scales as approximately n³, whereas the size of a2-tier leaf-spine network scales only as n². Accordingly, leaf-spinenetwork architectures employing 3 tiers of network devices open up thepossibility of efficiently assembling even larger networks.

Other Embodiments

Note that in the foregoing disclosure, numerous specific embodimentswere set forth in order to provide a thorough understanding of theinventive concepts disclosed herein. However, it will be appreciated bythose skilled in the art that in many cases the disclosed concepts maybe practiced with or without certain specific details, such as by thesubstitution of alternative elements or steps, or by the omission ofcertain elements or steps, while remaining within the scope and spiritof this disclosure. Furthermore, where certain processes, procedures,operations, steps, elements, devices, modules, components, and/orsystems are already well-known to those skilled in the art, they may notbe described herein in as great of detail as is necessarily possible, sothat the inventive aspects of this disclosure are not unnecessarilyobscured. Furthermore, note that the foregoing disclosed processes,methods, systems, and apparatuses have been described in detail withinthe context of specific embodiments for the purpose of promoting clarityand understanding, it will be apparent to one of ordinary skill in theart that there are many alternative ways of implementing theseprocesses, methods, systems, and apparatuses which are within the scopeand spirit of this disclosure. Accordingly, the embodiments describedherein are to be viewed as illustrative of the disclosed inventiveconcepts rather than limiting or restrictive, and are not to be used asan impermissible basis for unduly limiting the scope of the appendedclaims.

The invention claimed is:
 1. A method of forwarding data over aninternet-protocol (IP) network, the method comprising: receiving, at anetwork device, an address resolution protocol (ARP) request packet froma source host connected to the IP network; identifying an IP address ofa destination host designated in the ARP request packet; determining, atthe network device, a location on the IP network where the destinationhost designated by the ARP request packet is connected, withoutreference to a media access control (MAC) address of the destinationhost specified in the ARP request packet received from the source host,by using location-identification information locally stored in adatabase on the network device; and forwarding the ARP request packet tothe location on the IP network, without broadcasting the ARP requestpacket on the IP network and without referencing the MAC address of thedestination host specified in the ARP request packet received from thesource host, wherein, the database is distributed across multiplenetwork devices including the network device and another network device,the location on the network is determined from the database afterforwarding the ARP request packet to the another network device, the IPnetwork comprises a leaf-spine network fabric, the network device is aleaf network device in the leaf-spine network fabric which is an initialnetwork device encountered by the ARP request packet when it reaches theIP network after it issues from the source host, thelocation-identification information comprises a list matching one ormore host IP addresses with one or more locations on the IP networkwhere the hosts are connected, and the location on the IP network isdetermined from the list.
 2. The method of claim 1, wherein the sourcehost is connected to one or more switches and/or routers of the IPnetwork.
 3. The method of claim 1, further comprising: applying anencapsulation to the ARP request packet; and removing the encapsulationfrom the ARP request packet before it reaches the destination host. 4.The method of claim 3, wherein the encapsulation is virtual extensiblelocal-area network (VXLAN) encapsulation.
 5. The method of claim 1,wherein the network device is a virtual switch device which operateswithin a virtualization layer running on a physical host, and whereinthe source host issuing the ARP request packet operates as a virtualmachine within said virtualization layer.
 6. The method of claim 1,wherein the network device is a leaf network device in the leaf-spinenetwork fabric.
 7. The method of claim 1, wherein the database includesthe list.
 8. The method of claim 1, wherein: the location on the IPnetwork is determined from the database before the ARP request packet isforwarded from the network device.
 9. The method of claim 1, wherein:the location on the IP network is determined from the list before theARP request packet is forwarded from the initial network device.
 10. Themethod of claim 1, wherein: the network device is a leaf network device,the list is distributed across multiple network devices including theleaf network device and a spine network device; and the location on theIP network is determined from the list after forwarding the ARP requestpacket from the leaf network device to the spine network device.
 11. Themethod of claim 10, wherein the leaf-spine network fabric is deployed ina data center.
 12. The method of claim 10, further comprising: applyingan encapsulation to the ARP request packet; and removing theencapsulation from the ARP request packet before it reaches thedestination host.
 13. The method of claim 12, wherein the encapsulationis applied by the initial network device; and wherein the encapsulationcarries a proxy IP address associated with a proxy-function networkdevice.
 14. The method of claim 13, wherein the encapsulation is VXLANencapsulation.
 15. The method of claim 13, wherein the ARP requestpacket is forwarded to a third network device which is said location onthe IP network where the destination host is connected.
 16. The methodof claim 15, wherein the third network device is another leaf networkdevice.
 17. The method of claim 15, wherein the third network device isa virtual switch device which operates within a virtualization layerrunning on a physical host, and wherein the destination host operates asvirtual machine within said virtualization layer.
 18. The method ofclaim 15, wherein the encapsulation is removed from the ARP requestpacket at the third network device; and wherein the method furthercomprises: labeling the ARP request packet with the MAC address of thedestination host, wherein the third network device labels the ARPrequest packet using a locally stored MAC address; and forwarding theARP request packet from the third network device to the destinationhost.
 19. The method of claim 1, further comprising: preparing an ARPresponse packet in reply to the ARP request packet using a locallystored MAC address for the destination host; and forwarding the ARPresponse packet to the initial network device encountered by the ARPrequest packet when it reached the IP network after it issued from thesource host.
 20. The method of claim 3, further comprising: removing theencapsulation from the ARP request packet at a network device which issaid location on the IP network where the destination host is connected;and broadcasting the ARP request packet from the network device which issaid location on the IP network where the destination host is connectedto the destination host.
 21. The method of claim 3, further comprising:removing the encapsulation from the ARP request packet which is saidlocation on the IP network where the destination host is connected; andlabeling the ARP request packet with the MAC address of the destinationhost, wherein the network device which is said location on the IPnetwork where the destination host is connected labels the ARP requestpacket using a locally stored MAC address; and forwarding the ARPrequest packet from the network device to the destination host.
 22. Themethod of claim 19, wherein the ARP response packet is prepared by aleaf network device connected to the IP network and the destinationhost.
 23. The method of claim 22, wherein the ARP response packet isprepared in reply to the ARP request packet without notifying a targetdevice.
 24. A network device for receiving an address resolutionprotocol (ARP) request packet from a source host connected to an IPnetwork and forwarding the ARP request packet to a destination hostconnected to the IP network, said network device comprising:machine-readable instructions for: receiving the ARP request packet fromthe source host connected to said network device; identifying an IPaddress of the destination host designated the ARP request packet;determining a location on the IP network where the destination hostdesignated by the ARP request packet is connected, without reference toa media access control (MAC) address of the destination host specifiedin the ARP request packet received from the source host, via look-up ofa destination IP address in a list of location-identificationinformation locally stored in a database on the network device; andforwarding the ARP request packet to another network device withoutbroadcasting the ARP request packet on the IP network and withoutreferencing the MAC address of the destination host specified in the ARPrequest packet received from the source host; and circuit logic and/or aprocessor configured to execute the instructions, wherein, the databaseis distributed across multiple network devices including the networkdevice and another network device, the location on the IP network isdetermined from the database after forwarding the ARP request packet tothe another network device, the IP network comprises a leaf-spinenetwork fabric, the network device is a leaf network device in theleaf-spine network fabric which is an initial network device encounteredby the ARP request packet when it reaches the IP network after it issuesfrom the source host, the location-identification information comprisesa list matching one or more host IP addresses with one or more locationson the IP network where one or more hosts are connected, and thelocation on the IP network is determined from the list.
 25. The networkdevice of claim 24, wherein the list of location-identificationinformation matches one or more host IP addresses with one or morelocations on the IP network where the one or more hosts are connected,the list associated with the network device.
 26. The network device ofclaim 25, wherein the database is distributed across multiple networkdevices including the network device and the another network device. 27.The network device of claim 24, further comprising logic for applying anencapsulation to the ARP request packet.
 28. The network device of claim27, wherein the encapsulation is VXLAN.
 29. The network device of claim24, wherein the network device is a leaf network device in theleaf-spine network fabric.
 30. The network device of claim 24, whereinthe network device is a spine network device in a leaf/spine networkfabric.
 31. An IP network comprising: a first set of multiple networkdevices for connecting multiple hosts to the IP network; and a secondset of multiple network devices for connecting together the first set ofmultiple network devices; wherein each network device in the first setof multiple network devices comprises logic for: receiving an addressresolution protocol (ARP) request packet from a source host connected tosaid network device; identifying an IP address of a destination hostdesignated in the ARP request packet; determining a location on the IPnetwork where the destination host designated by the ARP request packetis connected, without broadcasting the ARP request packet on the IPnetwork and without referencing a media access control (MAC) address ofthe destination host specified in the ARP request packet received fromthe source host, via look-up of a destination IP address in a first listof location-identification information locally stored in a database;labeling the ARP request packet with the location, wherein the locationincludes a second MAC address; and forwarding the ARP request packet toanother network device in the second set of multiple network deviceswhen the location is not the same network device having received the ARPrequest packet via the logic; and wherein, each network device in thesecond set of multiple network devices comprises logic for: receivingthe ARP request packet from the first set of multiple network devices;and forwarding the ARP request packet to the first set of multiplenetwork devices which is the location of the destination host on the IPnetwork, the database is distributed across multiple network devicesincluding the network device and another network device, the location onthe IP network is determined from the database after forwarding the ARPrequest packet to the another network device, the IP network comprises aleaf-spine network fabric, the network device is a leaf network devicein the leaf-spine network fabric which is an initial network deviceencountered by the ARP request packet when it reaches the IP networkafter it issues from the source host, the location-identificationinformation comprises a list matching one or more host IP addresses withone or more locations on the IP network where the hosts are connected,and the location on the IP network is determined from the list.
 32. TheIP network of claim 31, wherein the database is distributed acrossmultiple network devices including the network device and the anothernetwork device.
 33. The IP network of claim 31, wherein the networkfurther comprises: a third set of one or more network devices forproviding locations of destination hosts on the network; and whereineach network device of the third set of one or more network devicescomprises logic for: receiving another packet from a network device ofthe second set of multiple network devices; identifying the IP addressof a destination host designated in the another packet; determining thelocation on the IP network where a destination host designated by theanother packet is connected, without reference to the MAC address of thedestination host specified in the another packet, via look-up of thedestination IP address in a second list of location-identificationinformation stored on the IP network; labeling the another packet with alocation determined via the logic; and forwarding the another packet toa network device in the second set of multiple network devices.
 34. TheIP network of claim 33, wherein the first list oflocation-identification information and the second list oflocation-identification information match one or more host IP addresseswith one or more locations on the IP network where the hosts areconnected.
 35. The IP network of claim 34, wherein each network deviceof the first set of multiple network devices further comprises logicfor: forwarding the ARP request packet to a host whose said location onthe IP network is that of said network device.
 36. The IP network ofclaim 35, wherein each network device of the first set of multiplenetwork devices further comprises logic for: applying an encapsulationto the ARP request packet after receiving it via the logic; and removingthe encapsulation from the ARP request packet before forwarding it viathe logic.
 37. The IP network of claim 36, wherein the encapsulationapplied via the logic and removed via the logic is VXLAN.
 38. The IPnetwork of claim 37, wherein the first set of multiple network devicesincludes a plurality of leaf network devices; and the second set ofmultiple network devices includes a plurality of spine network devices.